Elastomeric polyurethane resins, as is well known, are built up of so-called soft segments and hard segments. The soft segments are derived from a higher molecular weight reactant which contains Zerewitinoff active hydrogen atoms, generally a polyester polyol, polyether polyol or polycarbonate polyol, while the hard segments are the urethane or urea groups formed by the reaction of chain lengthening agents with the polyisocyanate.
The physical properties of polyurethane resins are primarily determined by the number and strength of the intramolecular and intermolecular hydrogen bridge bonds formed between the various hard segments. If an otherwise constant formulation of reactants is used, the quality of the elastomers is thus limited by the number and type of hard segments present.